A T.P.O was basically a mobile sorting office that sorted mail through the night to a very tight schedule i.e. station deadlines. Sometimes mail will be received at one station and have to be sorted by the next one, this is what makes the schedule tight.
The T.P.O dispatches mail directly to post towns where mechanized sorting offices may only dispatch to a distribution office. This is the T.P.O,s main function, To assure that all mail posted late will be delivered the next day in small villages as well as large towns and cities.
As an example, if you were to post a letter in London at 17:00 addressed to Barnstaple in Devon, It would be taken to the sorting office then sorted and dispatched to the distribution office for Devon (Exeter), they would then have to sort the letter again and send it on to Barnstaple. If the letter travelled this way the chances of it making first delivery the next day would be slim.
Using the T.P.O,s helped to speed up the process by having the mail sorted on route. This allowed sorting offices to leave dispatching until later and utilizes time which is usually dead (Whilst mail is on the move).
Royal Mail has over the last 20 years or so gone into Automation and back to the road in a big way, with the use of sorting machines and a fleet of articulated lorries so the T.P.O became very expensive and did not meet the requirements of Royal Mail.